Energy giant E.on has been forced to hand back a record £12 million to vulnerable people after it was found guilty of breaking energy sales rules.

The mis-selling occurred between June 2010 and December 2013 and today energy watchdog Ofgem accused E.on of misleading hundreds of thousands of customers on the doorstep and over the phone.

The energy watchdog said that E.on failed to properly train and monitor its staff and those it employed through third-party telesales agencies.

While Ofgem’s investigation found no evidence that E.on’s senior management set out to deliberately mis-sell to customers, it did find that management did not do enough to identify issues or act on problems when discovered.

The mis-selling hit some 33,000 E.on customers who are normally recipients of the Warm Home Discount. Each will be handed back £35 each.

Meanwhile the company has been forced to write to about 465,000 customers it has identified through its redress work, to tell them how to get in touch to find out whether they were mis-sold.

Which? executive director Richard Lloydsaid: “Our investigations in 2011 found clear evidence that suppliers weren’t giving people the best deal, and it’s extraordinary that mis-selling has apparently continued for so long since Ofgem began its own investigation.

“Energy suppliers should not wait for the outcome of the proposed competition review and must seize their chance to sort out woeful service standards and put customers first.”

Tom Lyon, an energy expert at uSwitch.com, added: “E.on’s behaviour is disappointing, particularly given the length of time these poor practices went on for and the fact that they were happening as recently as last December.

“Trust in the industry is already at an all-time low and this fine will only serve to further increase consumers’ cynicism towards energy suppliers.”

E.ON will make automatic payments to vulnerable customers listed on its Priority Service Register who were potentially mis-sold to during the investigation period.

But if you think you may have been mis-sold call E.on’s dedicated phone number on 0800 0568 497.